The present invention relates to the field of flexible wiring boards, particularly to the technique of laminating flexible wiring board pieces into a flexible wiring board of multilayer structure.
Recently, multilayer flexible wiring boards are widely used in the field of electronic apparatus. A method for forming a multilayer flexible wiring board involves laminating a plurality of flexible wiring board pieces.
An example of process for laminating two flexible wiring board pieces is explained with reference to the attached drawings as follows. Referring to FIG. 8, the reference 110 represents a flexible wiring board piece used for lamination. This flexible wiring board piece 110 has a base film 117 on which a metal wiring 115 is formed by patterning a copper foil. A resin film 112 is provided on metal wiring 115.
This resin film 112 is thermoplastic, i.e. it is not adhesive at normal temperatures but softens to develop adhesiveness upon heating.
Resin film 112 has a plurality of openings 120 in which a plurality of bumps 121 are provided.
The bottom of each bump 121 is connected to metal wiring 115 while the top projects from the surface of resin film 112.
The reference 130 in FIG. 9 represents a flexible wiring board piece to be laminated to said flexible wiring board piece 110. This flexible wiring board piece 130 has a base film 132 on which a metal wiring 135 is formed by patterning a copper foil, similarly to said flexible wiring board piece 110.
A non-thermoplastic cover film 138 is provided on metal wiring 135 and said cover film 138 has openings 139 at the locations corresponding to bumps 121 of said flexible wiring board piece 110 so that the surface of metal wiring 135 is exposed at the bottom of each opening.
For laminating these two flexible wiring board pieces 110, 130, one flexible wiring board piece 130 is mounted on a table 142 with openings 139 upward and opposed to the other flexible wiring board piece 110 with bumps 121 downward (FIG. 10(a)).
Then, the top of bump 121 is contacted with the surface of metal wiring 135 at the bottom of opening 139, and an ultrasonic resonator 145 is pressed against base film 117 of one flexible wiring board piece 110 (FIG. 10(b)).
Ultrasonic resonator 145 is connected to an ultrasonic wave generator not shown, which is activated to transmit ultrasonic wave to ultrasonic resonator 145 pressed against flexible wiring board pieces 110, 130 so that ultrasonic resonator 145 applies ultrasonic wave to flexible wiring board pieces 110, 130. Thus, bumps 121 are connected to metal wiring 135 in contact with said bumps 121 by this ultrasonic vibration.
Then, these flexible wiring board pieces 110, 130 are heated under pressure so that resin film 112 softens to develop adhesiveness, whereby flexible wiring board pieces 110, 130 are connected via resin film 112 into a multilayer flexible wiring board 150 (FIG. 10(c)).
Thus, the use of ultrasonic resonator 145 eliminates the use of solder to electrically connect flexible wiring board pieces 110, 130 by a single ultrasonic application while pressing a plurality of bumps 121 against metal wiring 135.
However, such bumps 121 are formed by a complex copper plating process.
With such an ultrasonic resonator 145 pressed against the whole flexible wiring board pieces 110, 130 to apply ultrasonic wave, connection failure may occur if the heights of bumps 121 are not uniformity. For example, the reference 151 in FIG. 10(c) represents such a connection failure, in which a lower bump 122 is not connected to metal wiring 135 because of lack of contact with metal wiring 135 during ultrasonic application in contrast with a higher bump 121 connected to the surface of metal wiring 115.
Bumps 121 should have a uniformity of height within xc2x13 xcexcm or less to efficiently connect flexible wiring board pieces 110, 130, but this is difficult when bumps 121 are formed by electroplating as described above.
Moreover, bumps 121 should fill openings 120 and project from the surface of resin film 112 by growing copper to a thickness of 40 xcexcm or more, but it takes one hour or more to attain such a thickness by electroplating, resulting in increase of production costs.
An object of the present invention is to obtain a multilayer flexible wiring board from flexible boards at low cost with high yield to overcome the disadvantages of the prior art described above.
In order to attain the above object, the present invention provides a process for manufacturing a multilayer flexible wiring board by laminating flexible wiring board pieces having a base film including a resin film and a metal wiring provided on said base film, said process comprising applying ultrasonic wave to said metal wirings of said flexible wiring board pieces to be laminated in close contact with each other at their surfaces to bond said metal wirings.
The present invention also provides a process for manufacturing a multilayer flexible wiring board by bonding metal wirings of at least two flexible wiring board pieces having a base film including a resin film and a metal wiring provided on said base film, said process comprising contacting the tip of ultrasonic resonator with the exposed opposite side of a portion to be bonded of said metal wirings of at least one flexible wiring board piece in two flexible wiring board pieces to be bonded, and applying iltrasonic wave to said ultrasonic resonator to bond said two metal wirings to be bonded.
In the process for manufacturing a multilayer flexible wiring board of the present invention, ultrasonic wave is applied to said metal wirings in close contact with each other at their surfaces while a thermoplastic resin layer developing adhesiveness upon heating is placed between said metal wirings.
In the process for manufacturing a multilayer flexible wiring board of the present invention, said metal wirings are ultrasonically bonded and then heated to laminate said flexible wiring board pieces by the adhesion of said thermoplastic resin.
In the process for manufacturing a multilayer flexible wiring board of the present invention, a metal coating selected from a metal coating based on gold, a metal coating based on silver, a metal coating based on nickel, a copper-nickel alloy coating, a coating based on aluminium, a coating based on titanium and a solder coating is preliminarily formed on at least one of the surfaces of the parts of said metal wirings to be ultrasonically bonded before said metal wirings are ultrasonically bonded.
In the process for manufacturing a multilayer flexible wiring board of the present invention, ultrasonic wave is individually applied to the parts of said metal wirings to be bonded.
The present invention also provides a multilayer flexible wiring board formed by laminating at least two flexible wiring board pieces having a base film and a metal wiring provided on said base film, wherein at least one flexible wiring board piece has a cover film including a resin film on said metal wiring and a first opening is provided on said cover film, and said metal wiring exists at the bottom of said first opening so that said metal wirings of said flexible wiring board pieces are bonded to each other by applying ultrasonic wave while the part of said metal wiring located at the bottom of said first opening is in close contact with said metal wiring of the other flexible wiring board piece.
In the multilayer flexible wiring board of the present invention, said cover film has insulating properties to prevent said connected metal wirings from contacting with each other except for the part located at said first opening.
In the multilayer flexible wiring board of the present invention, said cover film has a thermoplastic resin layer developing adhesiveness upon heating at least on its surface.
In the multilayer flexible wiring board of the present invention, said first opening and said metal wiring located at the bottom of said first opening forms a concave and the part of said metal wiring of the other flexible wiring board piece to be bonded to said concave is convex on said base film.
In the multilayer flexible wiring board of the present invention, said convex part of said metal wiring of the other flexible wiring board piece has an area smaller than that of said first opening forming said concave.
The present invention also provides a multilayer flexible wiring board formed by laminating at least two flexible wiring board pieces having a base film and a metal wiring provided on said base film, wherein said base film of at least one flexible wiring board piece has a second opening in which said metal wiring exists at the bottom so that said metal wirings are bonded to each other by applying ultrasonic wave while said metal wiring of the other flexible wiring board piece is in close contact with said metal wiring located at the bottom of said second opening.
In the multilayer flexible wiring board of the present invention, said base film has insulating properties to prevent said metal wirings from contacting with each other except for the part located at said second opening.
In the multilayer flexible wiring board of the present invention, at least one of the surfaces of the parts of said metal wirings to be ultrasonically connected has a metal coating selected from a metal coating based on gold, a metal coating based on silver, a metal coating based on nickel, a copper-nickel alloy coating, a coating based on aluminium, a coating based on titanium and a solder coating.
In the multilayer flexible wiring board of the present invention, said metal wirings to be connected to each other have the same type metal coating on their surfaces.
According to the present invention as defined above, ultrasonic wave is applied to metal wirings in contact with each other at their surfaces to eliminate formation of bumps via electroplating, thus reducing failures due to non-uniformity of bump height.
A resin film is provided on at least one flexible wiring board pieces to be laminated, and the contact region of the metal wiring is exposed at the bottom of an opening formed in the resin film, whereby the resin film at the opening serves to guide the metal wiring of the other flexible wiring board piece into the opening. In this case, the metal wiring of the other flexible wiring board piece may be convex to fit in a concave formed by the opening to facilitate aligning.
Ultrasonic wave is applied under pressure with an ultrasonic resonator directly pressed against the opposite side of one of metal wirings (portion to be bonded of metal wirings) in contact with each other, whereby ultrasonic wave is individually applied to the contact regions to ensure electric connection between the contact regions.
Said resin film may be formed of a thermoplastic resin to mechanically connect flexible wiring board pieces by the adhesion of the resin film.
In case of ultrasonic bonding, the condition of bonding of metal coating is depend on the combination of metal materials forming the parts of to be bonded. Table 1 below shows the results of our investigations on the ultrasonic bonding strength of combinations of various metals.
In Table 1, xe2x80x9c2xe2x80x9d means strong bond, xe2x80x9c1xe2x80x9d means weak bond and xe2x80x9c0xe2x80x9d means no bond.
In case of the present invention, the bond strength may differ from the results of Table 1, because the parts to be bonded are the surfaces of metal wirings. For example, copper-copper combination shows strong bond in case of wire bonding but resulted in weak bond in the metal coating formed on the surface of metal wirings of flexible wiring board pieces as in the present invention.
Accordingly, suitable metal coatings formed on the surface of the metal wiring of the present invention include a metal coating based on gold, a metal coating based on silver, a metal coating based on nickel, a copper-nickel alloy coating, a coating based on aluminium, a coating based on titanium and a solder coating. Two of these metal coatings may be chosen and formed on the surfaces of metal wirings to be ultrasonically bonded.
The flexible wiring board piece and the multilayer flexible wiring board of the present invention have property of flexibility.